1. Field of the Invention
The present invention relates generally to a digital printing system and, more generally, to techniques for pulsing energy to print heads in a printer.
2. Related Art
Referring to FIG. 16, a block diagram of a thermal printing system 1600 is shown which illustrates features common to many thermal printing systems. A thermal printer 1602 typically contains one or more print heads 1604a-b, which contain linear arrays of heating elements 1606a-h (also referred to herein as “print head elements”) that print on an output medium 1608 by, for example, transferring pigment or dye from a donor sheet to the output medium 1608 or by activating a color-forming chemistry in the output medium 1608. The output medium 1608 is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements 1606a-h (which may number in the hundreds per inch), when activated, forms color on the portion of the medium 1608 passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
A thermal print head element is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of pigment to the output medium or the formation of color in the output medium. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy provided to the print head element may be varied by, for example, varying the amount of power provided to the print head element within a particular time interval or by providing power to the print head element for a longer or shorter time interval.
Some conventional methods for color thermal imaging, such as thermal wax transfer printing and dye-diffusion thermal transfer, involve the use of separate donor and receiver materials. The donor material typically has a colored image-forming material, or a color-forming imaging material, coated on a surface of a substrate and the image-forming material or the color-forming imaging material is transferred thermally to the receiver material (i.e., the output medium 1608). In order to make multicolor images, a donor material with successive patches of differently-colored, or different color-forming, material may be used. In the case of printers having either interchangeable cassettes or more than one thermal head, different monochrome donor ribbons are utilized and the multiple color planes of the image are printed successively above one another. The use of donor members with multiple different color patches or the use of multiple donor members increases the complexity and the cost, and decreases the convenience, of such printing systems. It would be simpler to have a single-sheet imaging member that has the entire multicolor imaging system embodied therein.
In International Application No. PCT/US02/15868 (which corresponds to U.S. patent application Ser. No. 10/151,432, cross-referenced above), entitled “Thermal Imaging System,” there is described a direct thermal imaging system in which one or more of the thermal print heads 1604a-b can write two colors in a single pass on the single print medium 1608. The printer 1602 can write these multiple colors by addressing two or more image-forming layers of the output medium 1608 at least partially independently from the same surface so that each color can be printed alone or in selectable proportion with the other color(s).
The above-referenced patent application discloses an electronic pulsing technique that makes this result possible without modulating the heating element power supply voltage. Generally, each line printing time is divided into many subintervals. For example, referring to FIG. 1, a graph 100 is shown which plots the voltage across a single print head element (such as any one of print head elements 1606a-h) over time. Line interval 104 is subdivided into a plurality of subintervals 106a-g. In each of the subintervals, each print head heating element (also referred to herein simply as a “print head element”) potentially receives an electrical pulse. In the particular example illustrated in FIG. 1, pulses 110a-d are provided in each of subintervals 106a-d. 
Furthermore, the line printing time 104 can be divided into two segments, each containing a portion of the subintervals, as shown by the graph 200 in FIG. 2. Line interval 204 is divided into two segments 208a and 208b. The first segment 208a includes subintervals 206a-g and the second segment includes subintervals 206h-v. The pulses 210a-d in the first segment 208a are given a larger pulse duty cycle (the pulse duty cycle being the fraction of a subinterval during which power is applied) than the pulses 210e-p in the second segment 208b. The pulse duty cycle determines the average power being applied to the print head element during the segment and is used to select a particular one of the image-forming layers in the output medium 1608, and therefore to select a particular color to print.
In some instances this method for controlling the print head may not be completely satisfactory. For example, in wide format thermal printers in which multiple print heads are used in tandem to provide a wider format print it has been found to be advantageous to employ “screening” techniques when stitching together the image segments from each print head to form the final wider print. Examples of techniques for performing such stitching are disclosed in the above-referenced patent application entitled “Image Stitching for a Multi-Head Printer.” It is not, however, possible to accomplish effective screening using the pulse patterns just described with conventional thermal print heads.
The reason for this difficulty is that a conventional thermal print head typically has one or a small number of “strobe” signal(s) that service(s) all print head elements in the print head. The strobe signal determines the pulse duty cycle, and as a consequence all or a significant fraction of the print head elements 1606a-d in print head 1604a have the same pulse duty cycle in each subinterval; similarly, all or a significant fraction of the print head elements 1606e-h in print head 1604b have the same pulse duty cycle in each subinterval. The pulse duty cycle, in turn, determines the image-forming layer being printed, as described in the above-referenced patent application entitled “Thermal Imaging System,” and therefore it follows that during each subinterval all or a significant fraction of heating elements 1606a-d are printing on the same image-forming layer of the output medium 1608. Therefore, at any moment in time all or a significant fraction of the heating elements 1606a-d are printing the same color. This condition precludes the use of screening patterns that call for some of the heating elements 1606a-d to be printing on one image-forming layer (and therefore printing one color) while other ones of the heating elements 1606a-d are printing on another image-forming layer (and therefore printing another color).
It has been found, however, that some useful screening patterns require the print heads 1604a-b to print in just this way. For example, in the above-referenced patent application entitled “Image Stitching for a Multi-Head Printer,” there is described a screening technique for use with a method for stitching image segments to make the stitching method more insensitive to any misregistration of the dots. In general, the technique disclosed therein introduces a pattern of time delays into the rows of the image so that the pixels do not lie on a rectangular grid. Instead, the pixels in a row have a repeated pattern of displacements from the nominal (default) position of the row in the transport direction (“down-web”). In one embodiment, for example, the first pixel in the row is undisplaced, the second pixel is displaced down-web by ⅓ of a row spacing, the third is displaced by ⅔ of a row spacing, the fourth is undisplaced, and the pattern repeats. There are, then, three types of pixels in the row. The first, fourth, seventh, etc., are undisplaced pixels, the second, fifth, eighth, etc., are displaced down-web by ⅓ of a row and the third, sixth, ninth, etc., are displaced down-web by ⅔ of a row.
The use of such patterns may reduce the dependence of printing density in the stitch on the registration of the pixels. Furthermore, such patterns can be used to improve the tolerance to misregistration of colored dots formed on an imaging medium that has multiple superimposed color-forming layers in different planes, such as where one or more color-forming layers are arranged on a first side of a transparent substrate and at least one color-forming layer is arranged on a second side of the substrate. However, the down-web displacement of the pixels may cause the first time segment of some pixels to overlap the second time segment of others, requiring that some pixels be supplied with a low duty-cycle strobe pulse at the same time that others are being supplied with a high duty-cycle strobe pulse. As described above, the use of a single or a small number of strobe signal(s) for all print head elements in a print head may make it impossible to provide such varying pulse duty cycles across print head elements in the same subinterval. What is needed, therefore, are improved techniques for performing screening in a printer that can write two colors in a single pass on a single print medium.
Note further that power is typically provided simultaneously to multiple print head elements in a print head. Ordinarily, the printer power supply is chosen to satisfy the “worst case” demand represented by the supply of power to all of the print head elements simultaneously. This typically results in the choice of a larger and more expensive power supply than would be required to fulfill the “average” power demand. Power supplies may be chosen to satisfy this peak power requirement even when the average power provided to the print head elements is low, as is the case, for example, when there are repeated segments with low duty-cycle printing. What is further needed, therefore, are improved techniques for performing screening in a printer to reduce the peak power requirements.